The present invention relates to an optical transmission apparatus which is operable stably without being effected by ambient temperature.
A prerequisite with an optical transmission apparatus is that a beam issuing from a semiconductor laser or like light source be incident to an optical fiber, which defines a transmission path, while being maintained at a predetermined intensity level despite any variation of ambient temperature. One approach to meet such a prerequisite is sensing and controlling the optical output of the light source itself, as disclosed in U.S. Pat. No. Re. 31,969. Another approach is maintaining the temperature of the light source itself constant, as shown and described in U.S. Pat. No. 4,338,577. Specifically, the light source output sense and control scheme is such that light emanating from the opposite side of a semiconductor laser to the side which is connected to an optical fiber is sensed by a photosensor to so control a bias current applied to the laser as to maintain the average optical input to the optical fiber constant. This kind of scheme, however, has a problem that the current threshold of the laser varies with temperature. On the other hand, the light source temperature control scheme is such that the temperature of a semiconductor laser is sensed by a temperature sensor to so control the current being fed to a light absorbing and radiating arrangement as to maintain the temperature of the laser constant. Generally referred to as a thermoelectric cooler, the heat absorbing and radiating arrangement is implement by those elements which use, among various thermoelectric effects known in the art, the Peltier effect. Various efforts heretofore made to implement high quality and stable optical transmission rely on either one of the above-stated two different approaches.
A problem with using any of the above two approaches as it is that the ambient temperature range in which an optical transmission apparatus is usable would be limited and, therefore, the apparatus would fail to desirably operate under severe environmental conditions. A semiconductor laser, for example, is operable only in a temperature range which is usually 0.degree. C. to 60.degree. C. Specifically, at temperatures above 60.degree. C., the tendency of the optical output to drive current characteristic to become saturated due to the temperature dependence of external differential quantum efficiency is noticeably aggravated and, in addition, the tendency of the current threshold to increase is sped up. These in combination prevents a high optical output from being maintained and thereby renders the apparatus unfeasible for use in high temperature environments. On the other hand, at temperatures below 0.degree. C. which are lower than the dew point, decision regarding a kink at a laser device level is difficult; a kink would cause the optical output to become unstable therearound. When use is made of the previously mentioned heat absorbing and radiating arrangement, the permissible difference between the ambient temperature and the temperature of a semiconductor laser is up to 40.degree. to 45.degree. C. Hence, assuming that the temperature of a semiconductor laser itself is 25.degree. C., the upper limit of permissible ambient temperatures is substantially 65.degree. C. to 70.degree. C. Further, since such an arrangement needs an extra current source of about 1 ampere and consumes substantial power, some implementation has to be provided to reduce power consumption as far as possible.